The present applicant has previously proposed a lift device for lifting and lowering a table by a hydraulic cylinder unit (Examined Japanese Utility Model Publication No. 6875/1983). FIG. 8 shows the proposed device 9, which has a pair of intersecting arms 93, 94 pivoted to each other at their midportions and arranged between the upper surface of a base 92 and the lower surface of a table 91 at each of the right and left sides of the device. The arms 93, 94 have base ends pivoted to the base 92 and the table 91, respectively. A roller 95 is rotatably attached to the other end of each arm. The rollers 95, 95 are rollable on rails 96, 96 provided on the upper surface of the base 92 and the lower surface of the table 91. The arms 93, 93 pivoted to the base 92 are interconnected by a connecting rod 89 and links 88. A hydraulic cylinder unit 97 has a piston rod 98 connected to the midportion of the connecting rod 89. The unit 97 has a cylinder 99 fixed to the base 92.
When hydraulic oil is supplied to the cylinder unit 97, the piston rod 98 moves up, pushing up the connecting rod 89. With the upward movement of the connecting rod 89, the intersecting arms 93, 94 connected by the links 88 to the rod 89 are raised with the rollers 95, 95 rolling on the rails 96, 96 toward the base-end side to lift the table 91 while holding the table 91 in parallel to the base 92.
The hydraulic oil is discharged from the cylinder unit 97 to lower the lifted table 91. This releases the arms 93, 94 from the pressure of the cylinder unit 97, permitting the arms 93, 94 to move toward a falling direction under gravity acting on the table 91 or by being pulled by the downward stroke of the piston rod 98, with the result that the table 91 descends while remaining parallel to the base 92.
With the table lift device 9 of the above construction, the raised level of the table 91 is limited by the length of the intersecting arms 93, 94.
Such a device is usually so designed that the arms 93, 94 are stowed under the table 91 so as not to project outward beyond the table 91. In order to elevate the highest raised level of the table 91, therefore, there arises a need to link pairs of intersecting arms 93, 94 one above another in a pantographic manner.
The intersecting arms 93,94 are raised through an approximately definite angle by a single projecting stroke of the piston rod 98 when the hydraulic oil is supplied to the cylinder unit 97.
The roller 91 bearing against the table 91 moves along a circular-arc path during the ascent of the table 91, so that as the arms 93, 94 approach from the position wherein they are parallel to the base 92 to a position perpendicular to the base, the rate of rise of the arms gradually decreases. For this reason, the amount of projection of the cylinder unit piston rod 98 is not in proportion to the amount of ascent of the table 91, and it is difficult to control the rate of ascent of the table 91.
An object of the present invention is to provide a lift mechanism which is adapted to lift the table at a higher level and to render the table movable upward and downward at a constant rate.